With the advent of the increased awareness of the need for pollution abatement, and abatement of atmospheric particulte pollution in particular with regards to this invention, there has developed a need for a transmissiometer which is both portable and accurate to measure atmospheric pollution such as smoke, dust, smog, fog, haze and the like.
By portable, it is meant that there needs to be a transmissometer which can be located and relocated within a matter of a few hours if necessary, and moreover that it be capable of operating off of its own small portable power source, such as batteries, independent of normal commerical electrical transmission line outlets.
By accurate, it is meant that there need be a transmissometer which measure substantially only the particulates in the atmosphere and not the gases occurring naturally in the atmosphere including water vapor. Water vapor varies the same as humidity varies throughout the day and night, and the inclusion of this factor into the output measurement when only particulate concentration is attempted to be measured would introduce unacceptable error into the output measurement. Also to be accurate such a transmissometer must be capable of discriminating against stray light (light from sources other than the transmissometer source).
An examination of the physical laws governing optical attenuation discloses that the resultant signal strength at a distal point of a polychromatic non-collimated light source is a function of the path length and the photon absorption and scatter by the matter in the path beam between the source and the distal point. Thus to accurately measure the attenuation due to particulates in the atmosphere with a smaller power source as much of the light as possible irradiated from the source should be collected by a measuring device located at the distal point. This can be partly achieved by focusing with classical optical lens; for example, the transmissometers used at airports in measuring visibility focus with optical lens.
There has long existed transmissometers at airports to measure visibility, particularly visibility in fogs. These transmissometers usually comprise a comparatively large source light, such as an airport landing light (which requires a large power input) emitting a beam of polychromatic light optically focused on a receiver located a short distance away, at the most a very few hundred feet away. To discriminate against stray light a large long bulky collimating tube is usually placed on the receiver. This necessitates heavy foundations, usually concrete, to keep the receiver focused on the source. Thus the large power requirements and heavy foundation negates the airport transmissometer for use as a portable transmissometer. Furthermore, such a device when used to measure particulates in the atmosphere errs greatly when going from daylight to darkness. This is due to its inability to discriminate to a large enough extent against sunlight.
To overcome some of these deficiencies in the measuring atmospheric particulate concentrations the use of some lasers as light sources has been investigated; for example, the helium neon plasma type laser has been investigated. These plasma laser type transmissometers suffer from their inability to produce lifht of uniform intensity. This results in the requirement that the beam to be used in measuring be chopped and part of it sent to a nearby electronic receiver and comparator to be compared with the part of the beam transmitted through the atmosphere to be measured. This necessity to divert the laser beam into two different paths and bring them back together for comparison, requires chopping equipment and extra optical devices, all of which require more time and labor in moving and setting up. Moreover, for any extended periods of testing a gas-discharge laser requires accessability to a commercial power source or an unacceptably large portable power source. This reduces portability.
Although the gas-discharge light source greatly reduces the amount of power needed when compared to a heated wire source (as used in conventional airport transmissometers) to produce the same irradiance (power density) at a distal point, nevertheless, this power requirement is still too large and there is still too much equipment to be moved and time spent in moving a gas discharge laser for it to be considered truly portable.
In order for there to be measurable quantities of energy at the distal point, a light source of high irradiance must be used. However, this source must be physically small for long-distance focusing purposes. Also, the source can only require a small amount of energy in order for the power source to be portable.
For accurate measurement of atmospheric particulates a transmissometer needs to discriminate against stray light such as the sun's radiation. Measurements should not be effected by time of day or night. It must also be designed such that the absorption of the light by the gases occurring naturally in the atmosphere, and especially the light absorbed by the water vapor in the atmosphere, has virtually no effect on the accuracy of the transmissometer's measurement of particulates in the atmosphere.
These criteria have been achieved by the present invention.